Typical patents showing various modes of fastening the center plate to the blades are: the Mayne, U.S. Pat. No. 3,055,578, under which the assignee of the present application was a licensee; the Baker, U.S. Pat. No. 3,711,914, owned by the assignee of this application; the Merz, U.S. Pat. No. 3,080,105; and, the Ranz, U.S. Pat. No. 3,737,966.
The last-mentioned patent has a single center plate, the outer edge of which has a plurality of slots therein to receive the inner edges of the blades. These slots formed by slitting the plate edge radially, and bending the metal on each side of the slot out of the plane of the plate. The inner edges of the blades are placed in the slots so formed and the bent edges of the slits are bent back into the plane of the plate to tightly grip the inner edges of the blade.
In the first-mentioned patent, the middle of the inner edge of each blade is provided with a notch. A pair of abutting discs form the center plate. These discs initially each have circumferentially extending axial bulges adjacent to but spaced from the outer edge such that when edges are placed in the notches and the bulges are flattened by forming dies, the outermost edges of the discs move axially apart and into pressure engagement with the sides of the notches. In the first-mentioned patent, the sides of the notches are generally parallel. In the patent to Merz, the sides diverge outwardly from the radial plane at an angle of about 8.degree. (scaling from the drawing)
The present invention is an improvement on the general type of structure shown in the Mayne, Baker and Merz patents wherein the center plate is made up of two abutting discs so shaped that when the bulges are flattened, the outer edges of the discs move outwardly and apart into engagement with the sides of the notches.
Centrifugal type blower wheels are often subjected to severe operating conditions for prolonged periods. For example, a blower wheel may run continuously for twenty to twenty-five years. During such time, they are subjected to continual mechanical stresses and vibration which can result in fatigue failure of the blade attachment to the center plate. Failure of a blower wheel can have serious consequences.
It is impractical to operate a blower wheel for such prolonged periods to determine if fatigue failures will result. However, it is believed that the chances of fatigue failure increase as the normal operating speed approaches the "bursting speed"., that is the rotational speed at which the blade-plate attachment failes. The quality of the attachment of the center plate to the blades can be rated by the bursting speed of the assembled wheel.
Bursting speed failure of the wheels using the double disc supporting plate shown in Merz results when the edges of the discs are forced toward each other by the wedging action of the notch side walls on the disc edges force the edges axially toward each other a sufficient amount that the inner corners of the notches can pass by the edges of the discs. In Mayne, there is only the friction of metal against metal to resist the radial forces.
A further way of rating blower wheels has been to determine the deflection of the wheel both axially and diametrically as the speed of the wheel is increased during a bursting speed test.
Another problem with blower wheels has been failure due to the center support plate slipping circumferentially relative to the blades. The ability of a blower wheel to resist slipping can be determined by a stationary slip-torque test wherein the blades are restrained from rotation and an increasing torque is imposed on the supporting plate until slippage actually results.
As the diameter and rotational speed of centrifugal type blower wheels increase, these problems of fatigue failure, deflection and slippage of the plate relative to the blades becomes an important factor in the design and manufacture of blower wheels.